SOUTH SAN FRANCISCO, Calif.--(BUSINESS WIRE)--Jun 2, 2008 - Cell
Genesys, Inc. (Nasdaq:CEGE) today announced interim data from the
ongoing Phase 1 clinical trial of GVAX immunotherapy for prostate
cancer used in combination with ipilimumab in patients with
advanced prostate cancer. Updated data reported includes median
follow-up of approximately 22.3 months on the 12 patients enrolled
in the escalation cohort of the trial and interim data on the 16
patients enrolled in the expansion cohort of the trial. These data
were presented on Saturday, May 31, 2008, by Winald Gerritsen,
M.D., Ph.D., director of the University Hospital Vrije Universiteit
Cancer Center in Amsterdam, at the annual meeting of the American
Society of Clinical Oncology (ASCO) being held in Chicago,
Illinois. GVAX immunotherapy for prostate cancer is an
investigational product currently in development by Cell Genesys
and ipilimumab is an investigational fully human anti-CTLA-4
antibody in development by Medarex, Inc. in partnership with
Bristol-Myers Squibb Company.

This Phase 1 clinical trial has now been enrolled with 28
patients, including 12 patients in the escalation cohort and 16
patients in the expansion cohort. As previously reported, of the 12
patients enrolled in the escalation cohort, anti-tumor activity was
observed in five of the six patients who received the two highest
doses of ipilimumab (3 mg/kg and 5 mg/kg), including
prostate-specific antigen (PSA) declines of greater than 50
percent. Four of these five patients maintained this PSA decline
for at least six months, with the longest response to date now
reported to be approximately 24 months. Of the 16 patients enrolled
in the expansion cohort of the trial, 15 have completed dosing with
the combination regimen with ipilimumab administered at 3 mg/kg. As
of this time in follow-up, seven patients experienced stable PSA
that was durable for a median duration of 5.6 months. Of the 12
patients with bone lesions at baseline, five have ongoing stable
disease, with the most durable reported to date at nine months.

"These updated results are encouraging with respect to the
overall level of anti-tumor activity observed across the two
cohorts of patients who received the combination of GVAX
immunotherapy for prostate cancer and ipilimumab in this Phase 1
trial," Robert J. Dow, MBChB, senior vice president of Medical
Affairs and chief medical officer of Cell Genesys. "Of particular
note was the duration of decreased or stable PSA and bone scan in
approximately half of these patients with advanced prostate cancer.
We plan to continue to follow the patients enrolled in the trial
and then, together with our collaborators, determine the next steps
for evaluating this combination immunotherapy."

Data from the Phase 1 trial suggest that side effects associated
with GVAX immunotherapy for prostate cancer administered in
combination with ipilimumab are manageable and most often include
low grade fever and fatigue. In addition, immune-mediated adverse
events similar in type to those previously reported with ipilimumab
administration were observed in this combination therapy trial. Of
particular note, patients who experienced PSA declines of greater
than 50 percent generally experienced immune-mediated endocrine
deficiency, referred to as hypophysitis, which is comparable to
that previously reported with ipilimumab administration and which
was successfully treated with standard hormone replacement
therapy.

The Phase 1 trial of GVAX immunotherapy for prostate cancer used
in combination with ipilimumab was designed to evaluate safety and
to determine a maximum tolerated dose of ipilimumab when used in
combination with GVAX immunotherapy for prostate cancer. Efficacy
endpoints include time to clinical disease progression, time to PSA
progression and PSA response, immune response to GVAX
immunotherapy, reduction in metabolic bone activity and survival.
The dose of GVAX immunotherapy for prostate cancer used in this
combination trial is comparable to that currently being evaluated
in Cell Genesys' ongoing Phase 3 program. In the escalation phase
of the trial, three patients were assigned to each of the four
ipilimumab dose cohorts (0.3 mg/kg, 1 mg/kg, 3 mg/kg and 5 mg/kg).
In the expansion phase of the trial, which enrolled a total of 16
patients, all patients were assigned to receive 3 mg/kg of
ipilimumab.

About GVAX Immunotherapy for Prostate Cancer

GVAX immunotherapy for prostate cancer is a whole-cell, non
patient-specific product designed to present the immune system with
a broad spectrum of tumor antigens and stimulate an immune response
against the patient's tumor. GVAX immunotherapy for prostate cancer
is comprised of two prostate tumor cell lines that have been
modified to secrete GM-CSF (granulocyte-macrophage
colony-stimulating factor), an immune stimulatory protein that
plays a key role in stimulating the body's immune response, and
then irradiated for safety. Cell Genesys, in partnership with
Takeda Pharmaceutical Company Limited, is currently evaluating GVAX
immunotherapy for prostate cancer in two Phase 3 clinical trials,
VITAL-1 and VITAL-2, for the treatment of advanced stage,
hormone-refractory prostate cancer. In 2007, the VITAL-1 trial
completed enrollment with 626 patients and in January 2008, Cell
Genesys announced that the Independent Data Monitoring Committee
(IDMC) had completed a pre-planned interim analysis for VITAL-1 in
the timeframe originally estimated and recommended that the study
continue. The company currently estimates that there will be
sufficient events to trigger the final analysis for VITAL-1 in the
second half of 2009. Patients are continuing to be enrolled in the
VITAL-2 trial at approximately 100 clinical trial sites located in
North America and Europe. Cell Genesys is targeting the completion
of enrollment for VITAL-2 with approximately 600 patients in the
first half of 2009 and expects that there will be sufficient events
to trigger the pre-planned interim analysis in the same time frame.
GVAX immunotherapy for prostate cancer is currently being
manufactured in Cell Genesys' bioreactor manufacturing facility, a
facility that is capable of producing the product during
commercialization.

About Ipilimumab

Ipilimumab, formerly referred to as MDX-010, is a fully human
antibody that binds CTLA-4 (cytotoxic T lymphocyte-associated
antigen 4), a molecule on T-cells that plays a critical role in
regulating immune responses. The absence or presence of CTLA-4 can
augment or suppress the immune system's T-cell response in fighting
disease. Ipilimumab is designed to block the activity of CTLA-4,
thereby sustaining an active immune response in its attack on
cancer cells. Ipilimumab is being developed through a joint
partnership between Medarex and Bristol-Myers Squibb. The two
companies are pursuing a broad clinical development program with
ipilimumab evaluating its potential use in advanced melanoma, as
well as prostate, lung, pancreatic, bladder, breast, lymphoma and
leukemia cancers. More than 2,000 patients have been treated with
ipilimumab as a monotherapy or in combination with other agents in
clinical trials. Further information regarding the ipilimumab
program can be found in Medarex's public disclosure filings with
the U.S. Securities and Exchange Commission (SEC).

About Cell Genesys

Cell Genesys is focused on the development and commercialization
of novel biological therapies for patients with cancer. The company
is currently pursuing two clinical stage product platforms-GVAX(R)
cancer immunotherapies and oncolytic virus therapies. Ongoing
clinical trials include Phase 3 trials of GVAX immunotherapy for
prostate cancer, which is being developed in partnership with
Takeda Pharmaceutical Company Limited, Phase 2 trials of GVAX
immunotherapies for pancreatic cancer and for leukemia, and a Phase
1 trial of CG0070 oncolytic virus therapy for bladder cancer. Cell
Genesys continues to hold an equity interest in its former
subsidiary, Ceregene, Inc., which is developing gene therapies for
neurodegenerative disorders. Cell Genesys is headquartered in South
San Francisco, CA and has its principal manufacturing operation in
Hayward, CA. For additional information, please visit the company's
website at www.cellgenesys.com.

Statements made herein about the company, other than statements
of historical fact, including statements about the progress,
results, findings and timing of the company's clinical trials and
preclinical programs, current and potential corporate partnerships,
the nature of product pipelines and anticipated operating results
and cash expenditures are forward-looking statements and are
subject to a number of uncertainties that could cause actual
results to differ materially from the statements made, including
risks associated with the success of clinical trials and research
and development programs, the regulatory approval process for
clinical trials, competitive technologies and products, patents,
the ability to raise capital, operating expense levels and the
ability to establish and retain corporate partnerships and other
risks. For information about these and other risks which may affect
Cell Genesys, please see the company's reports on Form 10-Q, 10-K,
and 8-K and other reports filed from time to time with the
Securities and Exchange Commission. The company assumes no
obligation to update the forward-looking information in this press
release.